Process for refining metals, in particular liquid pig iron, in oxygen converters with continuous control of the operative procedure

ABSTRACT

A process for refining metals, in particular liquid pig iron into steel, in stationary or rotating reactors, by top-blowing oxidizing gases, particuarly oxygen, onto the liquid metal or slag bath, by means of control and/or continuous adjustment, made by an electronic computer, of the flow rate of the refining gas blown with a lance, and/or, - in the alternative - of the distance between the delivery head of the said lance and the metallic bath surface, wherein, on the basis of determined characteristic process indexes, an instantaneous metallurgical parameter m for the functioning of the refining process is established and for each type of operation there are prescribed a curve of optimal variation of the said metallurgical parameter m as a function of the blowing time, and a proper allowance band, within which the actual instantaneous values of the said metallurgical parameter can oscillate around the before mentioned optimal curve, these data being stored into the memory of the before mentioned electronic computer; during a given type of refining and during the whole blowing period, the said computer continuously and automatically performing the calculation of the actual instantaneous value of the said metallurgical parameter m, determining the actual variation curve with the time thereof, said curve being continuously and automatically compared with the prescribed optimal curve and the instantaneous actual value of m being continuously brought back to the corresponding optimal value or, at least, being maintained in the interval of the said allowance band.

United States Patent 1191 Ramacciotti et al.

[ Nov. 12, 1974 [75] Inventors: Aldo Ramacciotti, Rome; GiancarloEminian, Taranto, both of Italy [73] Assignee: Centro SperimentaleMetallurgico S.P.A., Rome, Italy [22] Filed: June 29, 1972 [21] Appl.No.: 267,694

[30] Foreign Application Priority Data July 13, 1971 Italy 31604/71 [52]US. Cl. 75/60, 75/59 [51] Int. Cl. C21c 5/30 [58] Field of Search 75/60,59

[56] References Cited UNITED STATES PATENTS 3,720,404 3/1973 Carlson75/60 3,594,155 7/1971 Ramachandran. 75/60 3,598,386 8/1971 Murphy 75/603,700,429 10/1972 Ramachandran... 75/60 3,723,099 3/1973 Marukawa 75/603,533,778 10/1970 Nilles 1 75/60 3,485,619 12/1969 Maatsch.... 75/603,372,023 3/1968 Krainer 1 75/60 3,719,469 3/1973 Roessing 75/603,475,599 10/1969 Schwartzenberg 75/60 Primary Examiner-C. LovellAssistant Examiner-Peter D. Rosenburg Attorney, Agent, or Firm-Young &Thompson [571 ABSTRACT A process for refining metals, in particularliquid pig iron into steel, in stationary or rotating reactors, bytop-blowing oxidizing gases, particuarly oxygen, onto the liquid metalor slag bath, by means of control and- /or continuous adjustment, madeby an electronic computer, of the flow rate of the refining gas blownwith a lance, and/or,in the alternative-of the distance between thedelivery head of the said lance and the metallic bath surface, wherein,on the basis of determinedpharacteristic process indexes, an instan taneous metallurgical parameter m for the functioning oi the refiningprocess is established and for each type of operation there areprescribed a curve of optimal variation of the said metallurgicalparameter m as a fu nction o f the blowing time, w allowance band,within which the actual instantaneous values of the said metallurgicalparameter can oscillate around the before mentioned optimal curve, thesedata being w iin the memory of t e e qremsatie e199; tronic computeri dur irig a given type of refining and during the whole blowing period, thesaid computer continuously and automatically performing the calculationof the actual instantaneous value of the said ngtallurgical parameter nbdetermining the actual v ar: iatiodcur ve with the time thereof,5211512696 being continuously and automatically compared with theprescribed optimal curve and the instantaneous actual value of m beingcontinuously brought back to the corresponding optimal value or, atleast, being maintained in the interval of the said allowance band.

1 Claim, 4 Drawing Figures PROCESS FOR REFINING METALS, IN PARTICULARLIQUID PIG IRON, IN OXYGEN CONVERTERS WITH CONTINUOUS CONTROL OF THEOPERATIVE PROCEDURE The present invention covers a process for refiningmetals, in particular liquid pig iron, in oxygen converters withcontinuous control of the operative procedure. More particularly theinvention relates to a liquid pig iron refining process, where aninstantaneous determination of the refining pattern is performed as wellas a continuous bringing back to a pattern preestablished as a standardmodel to be attained for the type of heat, by monitoring the oxygeninstantaneous balance, for the purpose of keeping all the heats withtemperature percentage of the final C and iron yield, within theallowance limits preestablished for each type of heat.

In the refining processes of pig iron in top-blown converters, theamount of oxygen blown under pressure and the distance between the lanceand the metal bath, assume great importance as operative controlvariables for the progress of the processes themselves. These values, infact, are those which determine the composition and the temperature ofthe resulting gas jet which impinges on the bath and the depth of itspenetration into the slag layer and the liquid metal, by influencing, inthis way, the pattern of the metallurgical refining process.

As a rule, both the distance lance-metal bath and the flow of the oxygenjet, are preestablished before the beginning of the refining, on thebasis of considerations suggested by the experience and modified, duringthe refining itself, on the basis of the variation of some values, suchas for example the intensity of some frequencies of the noise of theconverter, or on account of the subjective impressions of the operatingstaff. This kind of operation which requires a great deal of attentionand of skill from the staff, turns into an actual uncertainty as far asthe obtainable results are concerned; in fact, the variation of theoperational values considered, affects in various ways the trend of therefining, depending on the conditions existing in the converter, such asthe temperature and composition of the bath and of the slag, the amountof slag, etc. It is clear then, that in a situation like this, (whereall the values are variable, and vary in a different way, depending onthe general conditions existing in the converter), modification of someparameters without any possibility of an objective control, does notinsure the required reliability in carrying out the refining process.

In order to obviate these drawbacks, refining processes in which anobjective control of the operational procedure is effected, have beenproposed to this purpose. Substantially, mathematical models allowingthe determination of the optimal course of the process have beenproposed.

A model proposed by the Italian Pat. No. 766,770 is based on thefollowing equation:

at the stack. By integration of the equation, the amount of oxygen to beblown, c, in order to reach the required final value of carbon in thebath, is calculated a at a certain instant approaching the end point. Atthe same time, by means of another empirical model, not amenable to asimple formula. which expresses the var iation of the bath temperatureas a function of the amount of blown oxygen, the amount of oxygen 0still to be blown, in order to reach the final required temperature, iscalculated. If 0, 0 everything is proceeding normally; if instead O 0,one must intervene either adding coolants or modifying the distancebetween the lance and the bath.

The operational procedure proposed in the patent under consideration,presents a number of limitations, among which the most significant arethe following:

the analysis of the flue gases at the stack supplies data affected byerrors and it is not possible to determine, with sufficient precision,the various values and especially the constant k.

in the case where 0 is different from 0 the proper corrections of theprocess conditions are performed, but in so doing the value of theconstant k, calculated before the corrections, is also modified;therefore, in order to make the model correspond to the actual processconditions, it would be necessary to recalculate the new k value andconsequently the new 0 and O values, but in pratice theie is nosufficient time to perform these calculations and the furthercorrections of the process conditions.

the fundamental parameter on which the control and the specificdecarburization velocity is based; in other words, the model takespractically into account the decarburization reaction, while the oxygensupplied through the lance, besides reacting with the carbon of thebath, reacts also with the other present slag-forming elements, as wellas with the carbon monoxide.

Other techniques have been proposed, in order to make it possiblemonitoring the refining pattern, but for some of them, as in thetechnique mentioned above as an example, an intervention is planned bythe end of the refining, aiming at determining the amount of oxygenstill to be blown, in order to attain the required conditions. Obviouslythese techniques cannot be reliable, insofar as they do not work duringmost of the refining process and they are put into operation only when ashort time is still left for influencing with a certain effectiveness.

Furthermore, the control of the refining pattern is performed by meansof an analysis of the fumes at some location of the fume collecting anddepuration system, far away from the converter and generally downstreamof the waste heat boilers, and from these data a determination of thefumes analysis at the converter mouth is effected. Thesemethods,however, are made questionable both by the large amounts of nitrogenpresent in the analyzed gases, coming from the air drawn into the hoodand by the uncertainty inherent to the determination of the extent ofthe combustion reactions of CO into CO which occur outside of theconverter.

The object of the present invention is a refining process for metals,particularly for molten pig iron, in stationary or rotating oxygenconverters, which allows to obviate the afore mentioned limitations, bycontinuously monitoring the refining process.

The process, which is the object of the present invention, ischaracterized by an uninterrupted control'technique of the refining, inwhich recourse is made to a dynamic model based on the instantaneousbalance of the oxygen blown through the lance, calculated by means ofthe analysis of the discharge gases at the hood and at the stack, takinginto account that the said oxygen reacts not only with the carbon of thebath, to form carbon monoxide, but also with other elements present inthe bath, to form slag, and also, in the region above the bath, with thecarbon monoxide obtained in the initial reaction, to form carbondioxide.

In order words, the following three reactions are taken intoconsideration:

a. /20 C C c. 0 Fe, Si, Mn. .=Slag which represent respectively thedecarburization reaction of pig iron, the combustion reaction of carbonmonoxide and the slag forming reaction.

Each of these three reactions, considered separately, yield essentialinformation for the process control. Reaction (a) indicates how thedecarburization is proceeding; reaction (b) allows to know how thethermal yield of the conversion itself is varying in the course of therefining; reaction (c) gives a measure of the amount of slag which isforming and this is of a great interest as far as the regularity of therefining and the control of the metallic yield are concerned.

Actually, as it will be seen subsequently, only reactions (a) and (b)are used directly in the oxygen balance instantaneous calculation, sincethe amount of oxygen forming the slag is obtained by subtracting theamounts which react according to (a) and (b) from the total amount ofblown oxygen. However, the partial balance according to (c) isimportant, in that it explains why, in some periods of the blow, moreoxygen is used for reactions (a) and (b) than the amount blown (whichmeans that a portion of the slag previously formed, is decomposing,bringing back into the liquid metal bath, the corresponding portions ofiron, silicon, manganese, etc. mainly iron which previously had formedthe slag).

According to the present invention, a metallurgical reference parameteris determined, as a function of the reactions occurring inside theconverter during the refining, and of the amount of oxygen blown. Thesaid reference parameter is defined by the following instantaneousexperimental relationship:

where A instantaneous flow rate of the oxygen employed for thecombustion of C into CO, in Nm /min.

B instantaneous flow rate of the oxygen used for the combustion, insidethe converter, of CO to CO in Nm /min.

Q instantaneous flow of the total oxygen blown through the lance in Nmlmin K a constant characteristic of the type of refining.

tionship cannot be used to establish in advance a blowing pattern.

It must be observed that even though the distance of the lance from thebath is a control variable'very important for the development of therefining reactions, it does not appear explicitly in the formula whichdefines parameter m; that is due to the fact, experimentally observed,that the refining process reacts with a considerable inertia to theheight variations of the distance between the lance and the bath, whilethe response to flow rate variations is much prompter.

In order to utilize the metallurgical parameter as above defined,according to the present invention optimal curves are experimentallyestablished, indicating the variations of the said parameter m as afunction of the blowing time, each one corresponding to one of thevarious types of refining which are planned for execution, then for eachcurve, a range or variability band is established where the actualvalue' of m can oscillate without any negative effect on thesatisfactory progress of the heat. These data are stored in the memoryof an electronic computer.

For an actual refining, the type of the charge and the characteristicsrequired for the steel at the end point are preestablished. On. thebasis of these data the com puter chooses the relevant type of optimalcurve and, during the blowing, receives the data which allow it tocalculate A and B and thence the ratio K (A B)/Q m. Then it compares thevalue of this ratio at eac instant of the blowing to the optimal valuepreestablished for this instant, and if necessary, modifies the flowrate Q in such a way that the actual value of m be the nearest possibleto the optimal value, or at least, be comprised within thepreestablished variability range.

In this way a considerable progress is insured over the known art inthis technological field, where the chosen metallurgical parameters didnot allow, as previously mentioned, to obtain reliable results, or whereseparated consideration and evaluation of different variables eventhough collectively combined in the herein mentioned parameter m didnecessarily impose an arduous correlation of several data.

The methods of sampling and analyzing the gases and thepalculation modelfor A, B and margde s cribgd hereunder.

The refining control is performed by means of a continuous series ofsamplings and analyses of the gases issuing from the converter, for thepurpose of ascertainmg:

the composition of the gases (CO +CO2) at the converter mouth, beforetheir combustion with air. the composition of the gases at the stack ofthe steel ShOp. ()2 the measure of the pressure P, of the pressure dropAP, of the temperature T, of the flue gases of the stack for thecalculation of the flow rate of the gases in the dry state in Nm /sec.

According to the present invention, the analysis of the gases at theconverter mouth is performed by an indirect measurement, by sampling ofthe gas with a sampler within the hood, at adistance from the convertermouth ranging from 0.3 to 2.5 meters. A long series of tests has shownthat, in the hood, at the above mentioned distances from the convertermouth, only the outer zone is strongly concerned with the variations ofcomposition due to air entry, while in the central zone these variationsare minimal, thus causing errors which are within the required precisionlimits. The sampler, which can be tilted from 0 to 90 in respect to thehorizon, is thus introduced into the hood, in such way that its suctionextremity is located approximately in the center of the hood itself, ata distance of 0 to 0.5 meters from the axis of the hood itself, and at adistance from the converter mouth ranging from 0.3 to 2.5 meters.

According to the present invention, it is also possible to perform thesampling of the gases at a distance from the converter mouth, lesserthan 0.3 meters or even inside the converter itself, by means of asampler directly incorporated in the blowing lance or attached to it, orseparated from it and introduced into the converter mouth; however thepreselected technique (that is the sampling of the gases in the hood) isto be preferred as being the most practical, since no further liftingapparatus for the sampler is necessary, and the chance that a spray ofslag or liquid metal plugging the sampler is less probable; besides, thecomposition differences between the gases sampled inside the converterand those sampled with the suggested method are not too important.

The gas sampled by the sampler is first depurated of dust and thenconveyed to a continuous analyzer, of a conventional type, for CO andCO2 analysis and then to another continuous analyzer, also of aconventional type, for the analysis of oxygen. The nitrogen percentagepossibly present is calculated as a complement to one hundred of thetotal of the C0, C02 and 02 per centages. The instantaneous values ofthe thus determined percentages, are recorded and simultaneously fed toan electronic computer.

In the stack, the gases are practically cold and saturated with watervapor, because of the countercurrent water cooling and are alreadydepurated of dust; they are sampled by a sampler and conveyed directlyinto an analytical system, similar to that installed in the hood, andthe analysis data thus obtained are also recorded and simultaneously fedto an electronic computer.

In the stack, by means of a venturimeter-and a series ofthermoresistances, measurements of the differential pressure AP, and oftemperature T, are also performed. These data too are recorded and fedinto the computer.

As soon as the blowing is initiated, the computer be gins to read, witha proper frequency, the values corresponding to the single measurementsmentioned above. In real time too the computer elaborates the input datacalculates the instantaneous value of m and compares it with the valuepreestablished for this particular type of refining at the correspondinginstant.

The calculation procedure is the following where the data a e indicatedwith:

AP the differential pressure at the stack in kg/m P absolute pressure atthe stack in kg/m T temperature of gases at the stack in C Q the oxygen.flow rate from the lance in Nm-"/min.

% COZF 1.532 7472+ 0.532 2.532 P 53.2)/(% co (TB- .2. Water vaporpressure in the gases at the stack, in

kg/m

P exp (23.43097 5260.704/7C 273) where 7C corresponds to the temperatureof the gases in the stack.

3. Specific weight of the gases saturated in water vapor at the stack,in kg/m 4. Flow rate of the dry gases at the stack, in Nm lmin Q 1 LF!P- P /10330 273/T+ 273 where K is the venturimeter constant in thestack. 5. Oxygen used for the combustion of C into CO, in Nm /min 6.Oxygen used for the combustion in the converter of CO into CO in Nm lminB CO /l00 X A 7. Parameter m m K A B/QO,

Then, the computer compares continuously, during the blowing, the actualinstantaneous value of m with that indicated by the preestablishedcurve, and gradually, by trials, prescribes the corrections to bepossibly made to the distance between the lance and the bath and/or,preferably, to the flow rate of the oxygen delivered by said lance, inorder to bring the instantaneous value of m back to the optimal value.

The present invention will be hereunder described in greater detail withreference to the embodiments supplied, in a non limitative way, in thefollowing examples, and with reference to the drawings where:

FIG. 1 shows an optimal curve 1 of the parameter m, as a function of theblowing time, with the relevant allowance band or strip preestablishedfor a certain type of charge and for a certain requested result anddelimited by the dashed lines 2 and 3.

FIG. 2 shows the actual curve 4 of m, related to the heat indicated inExample I, superimposed over the curve of FIG. 1.

FIG. 3 shows the actual curve 5 of m, relative to the curve of the heatindicated in Example 2 and superimposed over the curve of FIG. 1;

FIG. 4 shows the actual curve 6 of m related to the heat indicated inthe Example 3, superimposed over the curve of FIG. 1.

Some examples of practical embodiment of the pro-' cess, according tothe invention, are now following.

EXAMPLE 1 Characteristic data liquid pig iron 245.4 tons steel scrap88.6 tons lime l5.6 tons and the total volume of O to be blown 15,600 NmAfter the charge of liquid and solid pig iron and of steel scrap hasbeen effected, the blowing is initiated with the following blowingparameters values flow rate 700 Nm /min lance distance from the bath1.800 m Since the beginning of the blow the computer performs thecalculation of m, with a frequency of 3.84 sec, but the automaticcontrol of the blowing parameters initiates only at the 4th minute ofblowing.

During the first two minutes 13.6 tons oflime and 0.5 tons of ore areloaded into the converter.

At the 4th minute the automatic control initiates and consequently thelance is lowered down to 1.50 meters.

During the remainder of the blowing period, the lance position is keptconstant, while the oxygen flow rates are permitted to vary within aninterval ranging from 600 to 800 Nm /min.

At the 17th minute of the blowing, 2 tons oflime and 600 kg of fluoriteare loaded.

At the 19th minute the oxygen flow rate is 800 Nm /min, the lancedistance from the bath is still 1.50 meters. The automatic control isinterrupted at about 2 minutes before the end point.

As can be seen in FIG. 2, the actual parameter m remained within theoptimal interval. The results at the end point were the following:

Bath temperature l605C 71 carbon in the bath 0.054

"/2 Mn do. 0.23

% P00 in the slag l7.2 Steel weight at the end point 328 tons Iron yield95.7 7:

The iron yield, at the allowance limits, is due to a sparking periodduring the first 3 minutes of blowing.

EXAMPLE 2 Liquid pig iron T l300C; '/r Analysis C 4.52

Si 0.53, Mn 0.88; S 0.039. P 0062 Solid pig iron 7.0 tons Iron ore 4.6tons Conditions required at the end point:

I600 i 10C Iron yield; )5 "i slag hasiuity index 4.0

The on-line computer on the basis of the previous data has indicated thefollowing compositions of the charge:

Liquid cast iron 272.6 tons scrap 57.9 tons lime 14.0 tons and theoxygen total volume to be blown 15,000 Nm After the solid and liquid pigiron and steel scrap have been loaded, the blowing is initiated with thefollowing values of the blowing parameters:

0 flow rate 650 Nm"/min Lance distance from the bath 1.80 m

During the first 3 minutes of blowing all the lime, the ore and 200 kgof fluorite are loaded.

From the 4th minute, the automatic control is initiated. The course ofthe parameter m during the refining is given in FIG. 3.

A slight slopping occurred from 6 to 6'40 of the blowing.

The final data after 20 of blowing are:

T of the bath 1602C 71 S 0.020 Steel weight 3l2.3 tons lron yield 96.271

72 FeO in the slag 20% For the purpose of stressing the advantagesconnected with the method proposed by the present invention, a series ofheats was performed in which the computer recorded the course ofparameter m during the blowing. but did not order the correctionswhereas the control of the refining was effected by hand following thesubjective impressions of the staff responsible for the heat. An exampleof heat performed in this way is given hereunder:

EXAMPLE 3 Characteristic data Liquid pig iron T= l,270C; Analysis:

C =4.5l Si =0.78; Mn =0.8 S 0.032; P 0.063 Iron in pigs 12.3 tons ironore 2.5 tons Required conditions at the end point:

Bath temperature l600C l0C 71 C 0.060 t 0.020 Iron yield a 71 Slagbasicity index 3.5

The on-line computer, on the basis of the previous data, indicated thecharge composition as follows:

Liquid pig iron 270.2 tons Scrap 53.0 tons Lime 17.8 tons and the oxygentotal volume to be blown 15,700 Nm After the solid and liquid pig ironand-steel scrap have been loaded, the blow is initiated with thefollowing blowing parameters:

;- flow rate 700 Nm- /min Lance distance from the bath 1.8 m

The lime, the ore and 200 kg of fluorite were loaded during the first 3.

At the 4th minute the lance distance from the bath I was brought to 1.50meters. The oxygen flow rate was maintained constant until the 17thminute, and thereafter was brought to 800 Nm /min.

The curve showing the course of the parameter m during the blowing isgiven in FIG. 4.

The final data of the metal bath are:

"/r Mn 0.28

Steel weight 298.6 tons Iron yield 92.2 71

The present invention has been described with particular reference tosome specific and preferred embodiments thereof, but it is intended thatmodifications or variations may in practice be introduced therein,without departing from the scope of the present invention as defined bythe appended claims.

Having thus described the present invention, what is claimed is:

1. In a process for refining liquid pig iron into steel in a liquidbath, comprising blowing a gas containing oxygen onto the bath, andanalyzing the gas above the bath; the improvement comprising the stepsof predetermining a desired graphical curve of m versus time on thebasis of at least the actual initial and desired final physicalcharacteristics of the bath, in which A being the instantaneous flowrate of the oxygen in said analyzed gas employed for the combustion of Cinto CO in Nm /min., B being the instantaneous flow rate of the oxygenin said analyzed gas used for the combustion of CO to CO in Nm /min.,and Q being the instantaneous flow rate of the total oxygen in saidblown gas in Nm /min., k being a constant; determining at a plurality oftime intervals the actual instantaneous values of m on the basis of thephysical characteristics of gas emitted from the bath; and varying theflow rate of said blown gas so as to decrease the difference betweeneach said actual value of m and the predetermined value of m at the samesaid time interval on said graphical curve until physicalcharacteristics close to said desired final physical characteristics ofthe bath are achieved.

1. IN A PROCESS FOR REFINING LIQUID PIG IRON INTO STEEL IN A LIQUIDBATH, COMPRISING BLOWING A GAS CONTAINING OXYGEN ONTO THE BATH, ANDANALYZING THE GAS ABOVE THE BATH; THE IMPROVEMENT COMPRISING THE STEPSOF PREDETERMINING A DESIRED GRAPHICAL CURVE OF M VERSUS TIME ON THEBASIS OF AT LEAST THE ACTUAL INITIAL AND DESIRED FINAL PHYSICALCHARACTERISTICS OF THE BATH, I WHICH